1. Field of the Invention
The present invention relates to a programmable sequence controller capable of performing a desired sequence control by applying into a memory device a program which is made in accordance with a sequence circuit.
2. Description of the Prior Art
In a conventional programmable sequence controller, an output signal of an input contact is compared with a control command for examining whether the input contact satisfies a control condition. Such a comparison is performed as briefly shown in FIG. 1. The output signal of the input contact is compared with a control command TON by an AND gate GA1 for examining whether the input contact is in an ON state. When the input contact is in the ON state, the AND gate GA1 generates a ONE signal. An inverted signal generated from an inverter IN1, which inverts the output signal of the input contact, is compared with a control command TOP by an AND gate GA2 for examining whether the input contact is in an OFF state. When the input contact is in the OFF state, the AND gate GA2 generates a ONE signal. When one of the AND gates GA1 and GA2 generates a ONE signal, a flip flop is set so that a satisfaction or unsatisfaction of an OR logic circuit can be examined.
In a sequence control circuit, there is usually provided an AND logic circuit as well as an OR logic circuit. In order to examine a satisfaction or unsatisfaction of the AND logic circuit in the conventional programmable sequence controller, the AND logic function must be converted into an OR logic function by using DeMorgan's theorem. For example, assuming that a relay CR3 is to be energized if input contacts CR1 and CR2 are both in an ON state, this relationship may be expressed in Boolean form as follows: EQU CR3 = CR1.CR2 (1)
by use of DeMorgan's theorem, the expression (1) can be converted to the following expression: EQU CR3 = CR1 + CR2 (2)
as can be understood from the expression (2), the control command TOF must be used in order to examine whether or not the input contacts CR1 and CR2 are in the ON state. Furthermore, the flip flop has to be set when examination of the input contacts CR1 and CR2 by the control command TOF are unsatisfied. For this reason, preparation of the program is troublesome and there is a possibility that the control commands TON and TOF will be misused. This troublesome programming is caused by the fact that only two control commands are provided for examining both the AND and OR logic functions, and that only one flip flop is provided to identify satisfaction or unsatisfaction of the examination for the AND and OR logic functions.